There are absolute indications for one-lung ventilation (e.g., hemothorax, unilateral lavage, unilateral cyst), but most procedures using double-lumen tubes are relative indications to facilitate surgical exposure. The adequate surgical exposure facilitates the dissection and reduces operative time.

During one-lung ventilation, the nondependent, nonventilated lung is excluded from the ventilation, with all the tidal volume (V_T) directed into the dependent lung. In this situation, the distribution of perfusion is the major determinant of the degree of venous admixture. The blood flow through the operative lung becomes a right-to-left shunt in addition to that which exists in the ventilated lung.<sup>15 Benumof J.L. Special respiratory physiology of the lateral decubitus position, the open chest, and one-lung ventilation Anesthesia for Thoracic Surgery Philadelphia: WB Saunders (1995).  123-151

Cliccare qui per andare alla sezione Riferimenti</sup> Given the same inspired oxygen concentration (Fio₂) and hemodynamic and metabolic status, one-lung ventilation results in a much larger alveolar-arterial oxygen tension difference P(a-a)o₂ and lower arterial oxygen partial pressure (Pao₂) than during two-lung ventilation.

In estimating the degree of shunt that is created by one-lung ventilation when it is performed in the lateral decubitus position, on average, 40% of cardiac output perfuses the nondependent lung and the remaining 60% perfuses the dependent lung (Figure 1).<sup>15 Benumof J.L. Special respiratory physiology of the lateral decubitus position, the open chest, and one-lung ventilation Anesthesia for Thoracic Surgery Philadelphia: WB Saunders (1995).  123-151

Cliccare qui per andare alla sezione Riferimenti</sup> Mechanisms that tend to decrease the percent of cardiac output perfusing the nondependent, nonventilated lung are passive (e.g., mechanical-like gravity, surgical manipulation, amount of pre-existing lung disease) or active (e.g., hypoxic pulmonary vasoconstriction).<sup>15 Benumof J.L. Special respiratory physiology of the lateral decubitus position, the open chest, and one-lung ventilation Anesthesia for Thoracic Surgery Philadelphia: WB Saunders (1995).  123-151

Cliccare qui per andare alla sezione Riferimenti</sup> The normal response of the pulmonary vasculature to atelectasis is an increase in pulmonary vascular resistance (in the atelectatic lung), and the increase in atelectatic lung resistance is almost entirely caused by hypoxic pulmonary vasoconstriction. Hypoxic pulmonary vasoconstriction is a protective reflex mechanism that diverts blood flow away from the atelectatic lung. With an intact hypoxic pulmonary vasoconstriction response, the transpulmonary shunt through the nondependent lung decreases to approximately 23% of the cardiac output (see Figure 1).